Operating System Lecture Notes - Introduction

In this note, you could find the basic definition of Operating Systems and the general architecture of computer.

Defining Operating Systems

What can an OS do for me?

• File systems: where is the file physically written on the disk and how is it retrieved?

• Abstraction: why looks the instruction the same independent of the device?

• Concurrency: what if multiple programs access the same file simultaneously? What if an other process starts running?

• Security: why is the access denied? Where in memory will the array be stored and how is it protected from unauthorised access?

• What if the array requires more memory than physically available?

• What if only part of the array is currently in use ?

What is part of the operating system?

Memory management, CPU scheduling, file system, communication, memory management, interrupt handling, GUI, . . .

A resource manager

• Many modern operating systems use multi-programming to improve user experience and maximize resource utilization
  • Disks are slow: without multi-programming, CPU time is wasted while waiting for I/O requests
    • Imagine a CPU running at 3.2 GHz (approx. 3:2 \(\times\) 109 instructions per second)
    • Imagine a disk rotating at 7200 RPM, taking 4.2 ms to rotate half a track
    • I/O is slow, we are missing out on 3.2 \(\times\) 4.2 \(\times\) \(10^6\) instructions (13.44m)!
• The implementation of multi-programming has important consequences for operating system design
• The operating system must allocate/share resources (including CPU, memory, I/O devices) fairly and safely between competing processes:
  • In time, e.g. CPUs and printers
  • In space, e.g., memory and disks
• The execution of multiple programs (processes) needs to be interleaved with one another:
  • This requires context switches and process scheduling ) \(\Rightarrow\) mutual exclusion, deadlock avoidance, protection, . .

Origin

In the early days, programmers had to deal directly with the hardware

• Real computer hardware is ugly
• Hardware is extremely difficult to manipulate/program

An operating system is a layer of indirection on top of the hardware:

• It provide abstractions for application programs (e.g., file systems)
• It provides a cleaner and easier interface to the hardware and hides the complexity of “bare metal”
• It allows the programmer to be lazy by using common routines :-)

Why study operating system?

• The programs that we write use operating system functionality
• How are the operating system’s services/abstractions implemented

Computer Architecture

Simplified computer model (Tanenbaum, 2014)

CPU design

• CPU’s basic cycle consist of fetch, decode, and execute (pipelines, or superscalar)
• Every CPU has his own instruction set
• A CPU has a set of registers (extremely fast memory close to the CPU “core”)
  • Registers are used to store data and for special functions (e.g. program counter, program status word – mode bit)
  • The compiler/programmer decides what to keep in the registers
  • Context switching must save and restore the CPU’s internal state, including its registers

image-20201004100845955

Memory management Unit

• There are two different address spaces:

  • the logical address space seen by the process and used by the compiler
  • the physical address space seen by the hardware/OS

• When compiling code, memory addresses must be assigned to variables and instructions, the compiler does not know what memory addresses will be available in physical memory

• It will just assume that the code will start running at address 0 when generating the machine code

  

• On some rare occasions, the process may run at physical address 0

  • physical address = logical address + 0

• On other occasions, it will be running at a completely different location in physical memory and an offset is added

  • physical address = logical address + offset

• The memory management unit(MMU) is responsible for address translation (“adding the offset”)

  • Different processes require different address translation (offsets)
  • Context switching requires the MMU to be updated (and registers, cache, ...)

Example

#include <stdio.h>
int iVar = 0;
void main() {
    int i = 0;
    while(i < 10) {
        iVar++;
        sleep(2);
        printf("Address:%u; Value:%d\n",&iVar, iVar);
         i++;
    }
}

• The same addresses will be displayed for iVar. The address printed on the screen is the logical address
• The value for iVar in the first run doesn’t influence the second run’s value

Moore’s Law

“The number of transistors on an integrated circuit (chip) doubles roughly every two years”

• Closely linked, but not necessarily related to performance
• Moore’s still continuing, but the “power wall” slows performance improvements of single core/single processor systems
  • A few cores for multiple “programs” is easy to justify
  • How to use massively parallel computers/CPUs/many core machines
  • Can we extract parallelism automatically, can we implement parallelism at the lowest level (similar to multiprogramming)

Lead to multi-core / parallel development

Multi-core, hyperthreaded processors

• Modern CPUs contain multiple cores and are often hyper-threaded

• Evolution in hardware has implications on operating system design

  • XP did not support multi processor architectures

  • Process scheduling needs to account for load balancing and CPU affinity

  • Cache coherency becomes important (manage run-time data)

    

Previous exam: Describe how, in your opinion, recent developments in computer architecture and computer design have influenced operating system design

Timer Interrupts

• Interrupts temporarily pause a process’s normal operation
• Different types of interrupts exist, including:
  • Timer interrupts by CPU clock
  • I/O interrupts for I/O completion or error codes
  • Software generated, e.g. errors and exceptions
• Context switches (i.e. switching between processes) can be initiated by timer interrupts after a “set time”

1. Timer generates an interrupt
2. CPU finishes current instruction and tests for interrupt
3. Transfer to interrupt service routine
   • Hardware saves current process state (PSW, program counter)
   • Set program counter to interrupt service routine
   • Save registers and other state information
4. Carry out interrupt service routine (scheduler)
5. Restore next process to run